As a secondary battery with high energy density, a nonaqueous electrolytic lithium secondary battery using a carbonaceous material as an anode is widely studied. While the demand for a nonaqueous electrolytic lithium secondary battery for use as a power supply of a mobile device is still increasing, a new use of a nonaqueous electrolytic lithium secondary battery as a battery of an electric vehicle such as an electric vehicle (EV) driven by a motor alone and a hybrid electric vehicle (HEV) using a combination of an internal combustion engine and a motor is actively developed.
A popularly-used constituent material of an anode of a lithium secondary battery is a carbon based material, and other examples include materials comprising a metallic element such as Zn, Al and Sn or a metalloid element such as Si, Ge, and Sb. As a carbon based material, non-graphitizable carbon (also called “hard carbon”) having a potential capacity that the discharged capacity per kilogram of carbon is significantly higher than the theoretical discharged capacity of graphite, 372 Ah/kg, is also widely used. Especially as a battery for an electric vehicle, non-graphitizable carbon has drawn great attention from the viewpoint of high input-output characteristics that high electric power is repeatedly supplied and received in a short period of time.
Non-graphitizable carbon using a petroleum-based or coal-based pitch as a raw material is proposed as suitable non-graphitizable carbon for use as a constituent material of an anode of a lithium secondary battery (Patent Documents 1 to 3). Typical, conventional steps of producing a desired anode material for a battery from a pitch material are shown in FIG. 1.
As shown in FIG. 1, in the existing process, an anode material is produced through the following steps of: melt blending a dicyclic or tricyclic aromatic compound having a boiling point of 200° C. or higher added as an additive to a petroleum-based or coal-based pitch material and molding to obtain a molded pitch (“melt blending and molding step”); extracting the additive from the molded pitch with a solvent having low solubility to the pitch and high solubility to the additive to obtain a porous molded pitch (“extracting and drying step”); oxidizing the porous molded pitch with an oxidizing agent such as air to obtain an infusibilized pitch (“oxidizing step” or “infusibilizing step”); heating the infusibilized pitch to 600° C. or 680° C. in an inert gas atmosphere (normal pressure) to remove an organic component (tar component) contained in the infusibilized pitch to obtain a carbon precursor having a low volatile content (“tar removal step”); grinding the carbon precursor to obtain a powdery carbon precursor (“grinding step”); and firing the powdery carbon precursor in an inert gas at about 800 to 1500° C. to carbonize the powdery carbon precursor (“heat treatment step”).
The porous molded pitch obtained through the step of melt blending and molding and the step of extracting has problems of dispersion of fine powder and inadequate removal of heat in the step of oxidizing. Hence, it is necessary to appropriately carry out a grinding or classification process to obtain a molded article with an appropriate size (average particle size: 100 to 2000 μm; the average particle size indicates a median size hereinafter, unless otherwise specified). Further, the use of an additive or extraction solvent in the step of melt blending and molding makes the production steps complicated because it requires the step of extracting and drying the additive or solvent afterward and, furthermore, the use of an additive or extraction solvent leads to emission of wastes, imposing problems in terms of production costs and environmental issues.
On the other hand, when a pitch material (average particle size: 0.07 to 10 mm) produced without being subjected to the step of melt blending and molding and the extraction step is subjected to infusibilization treatment as a petroleum based or coal based pitch material, the pitch material is not uniformly infusibilized, causing a problem that the infusibilization treatment takes a long time. Thus, an improvement has been demanded.